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Design considerations for Metadata Insertion

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8165.
Author Ted Hardie
Last updated 2017-02-21 (Latest revision 2017-01-20)
RFC stream Internet Engineering Task Force (IETF)
Stream WG state (None)
Document shepherd (None)
IESG IESG state Became RFC 8165 (Informational)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Stephen Farrell
Send notices to (None)
IANA IANA review state IANA OK - No Actions Needed
Network Working Group                                          T. Hardie
Internet-Draft                                          January 20, 2017
Intended status: Informational
Expires: July 24, 2017

              Design considerations for Metadata Insertion


   The IAB has published [RFC7624] in response to several revelations of
   pervasive attack on Internet communications.  This document considers
   the implications of protocol designs which associate metadata with
   encrypted flows.  In particular, it asserts that designs which do so
   by explicit actions of the end system are preferable to designs in
   which middleboxes insert them.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 24, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Design patterns . . . . . . . . . . . . . . . . . . . . . . .   2
   4.  Advice  . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Deployment considerations . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   5
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   To ensure that the Internet can be trusted by users, it is necessary
   for the Internet technical community to address the vulnerabilities
   exploited in the attacks document in [RFC7258] and the threats
   described in [RFC7624].  The goal of this document is to address a
   common design pattern which emerges from the increase in encryption:
   explicit association of metadata which would previously have been
   inferred from the plaintext protocol.

2.  Terminology

   This document makes extensive use of standard security and privacy
   terminology; see [RFC4949] and [RFC6973].  Terms used from [RFC6973]
   include Eavesdropper, Observer, Initiator, Intermediary, Recipient,
   Attack (in a privacy context), Correlation, Fingerprint, Traffic
   Analysis, and Identifiability (and related terms).  In addition, we
   use terms that are specific to the attacks discussed in [RFC7624].
   Terms introduced terms from there include: Pervasive Attack, Passive
   Pervasive Attack, Active Pervasive Attack, Observation, Inference,
   and Collaborator.

3.  Design patterns

   One of the core mitigations for the loss of confidentiality in the
   presence of pervasive surveillance is data minimization, which limits
   the amount of data disclosed to those elements absolutely required to
   complete the relevant protocol exchange.  When data minimization is
   in effect, some information which was previously available may be
   removed from specific protocol exchanges.  The information may be

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   removed explicitly (by a browser suppressing cookies during private
   modes, as an example) or by other means.  As noted in [RFC7624], some
   topologies which aggregate or alter the network path also acted to
   reduce the ease with which metadata is available to eavesdroppers.

   In some cases, other actors within a protocol context will continue
   to have access to the information which has been thus withdrawn from
   specific protocol exchanges.  If those actors attach the information
   as metadata to those protocol exchange, the confidentiality effect of
   data minimization is lost.

   The restoration of information is particularly tempting for systems
   whose primary function is not to provide confidentiality.  A proxy
   providing compression, for example, may wish to restore the identity
   of the requesting party; similarly a VPN system used to provide
   channel security may believe that origin IP should be restored.
   Actors considering restoring metadata may believe that they
   understand the relevant privacy considerations or believe that,
   because the primary purpose of the service was not privacy-related,
   none exist.  Examples of this design pattern include [RFC7239] and

4.  Advice

   Avoid this design pattern.  It contributes to the overall loss of
   confidentiality for the Internet and trust in the Internet as a
   medium.  Do not add metadata to flows at intermediary devices unless
   a positive affirmation of approval for restoration has been received
   from the actor whose data will be added.

   Instead, design the protocol so that the actor can add such metadata
   themselves so that it flows end-to-end, rather than requiring the
   action of other parties.  In addition to improving privacy, this
   approach ensures consistent availability between the communicating
   parties, no matter what path is taken.

   As an example, RFC 7871 describes a method that had already been
   deployed and notes that it is unlikely that a clean-slate design
   would result in this mechanism.  If a clean-slate design were to
   follow the advice in this document, that design would likely reverse
   a core element of RFC 7871: rather than adding metadata at a proxy,
   it would provide facilities for end systems to add it to their
   initial queries.  In the case of RFC 7871, the relevant metadata is
   relatively easy for an end system to derive, as STUN [RFC5389]
   provides a method for learning the reflexive transport address from
   which a client subnet could be derived.  By negotiating an EDNS0
   option which allowed them to self-populate this data, clients would
   be affirming their consent for its use and providing data at a

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   granularity with which they were comfortable.  This variability would
   change the caching behavior for responses from participating servers,
   but the same considerations set out in section 7.3.2 and 7.5 apply to
   client-supplied subnets as well as they do for proxy supplied

   From a protocol perspective, in other words, this approach would be a
   minor change from RFC 7871, would be as fully featured and would
   provide better privacy properties than the opt-in mechanism it
   provides.  The next section examines why, despite this, deployment
   considerations have sometimes trumped cleaner designs.

5.  Deployment considerations

   There are two common tensions associated with the deployment of
   systems which restore metadata.  The first is the trade-off in speed
   of deployment for different actors.  The Forwarded HTTP Extension in
   [RFC7239] provides an example of this.  When used with a proxy, it
   restores information related to the original requesting party, thus
   allowing a responding server to tailor responses according to the
   original party's region, network, or other characteristics associated
   with the identity.  It would, of course, be possible for the
   originating client to add this data itself, after using STUN
   [RFC5389] or a similar mechanism to first determine the information
   to declare.  This would require, however, full specification and
   adoption of this mechanism by the end systems.  It would not be
   available at all during this period, and would thereafter be limited
   to those systems which have been upgraded to include it.  The long
   tail of browser deployments indicates that many systems might go
   without upgrades for a significant period of time.  The proxy
   infrastructure, in contrast, is commonly under more active management
   and represents a much smaller number of elements; this impacts both
   the general deployment difficulty and the number of systems which the
   origin server must trust.

   The second common tension is between the metadata minimization and
   the desire to tailor content responses.  For origin servers whose
   content is common across users, the loss of metadata may have limited
   impact on the system's functioning.  For other systems, which
   commonly tailor content by region or network, the loss of metadata
   may imply a loss of functionality.  Where the user desires this
   functionality, restoration can commonly be achieved by the use of
   other identifiers or login procedures.  Where the user does not
   desire this functionality, but it is a preference of the server or a
   third party, adjustment is more difficult.  At the extreme, content
   blocking by network origin may be a regulatory requirement.  Trusting
   a network intermediary to provide accurate data is, of course,

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   fragile in this case, but it may be a part of the regulatory

   These tensions do not change the basic recommendation, but they
   suggest that the parties who are introducing encryption and data
   minimization for existing protocols consider carefully whether the
   work also implies introducing mechanisms for the end-to-end
   provisioning of metadata when a user has actively consented to
   provide it.

6.  IANA Considerations

   This memo makes no request of IANA.

7.  Security Considerations

   This memorandum describes a design pattern related emerging from
   responses to the attacks described in [RFC7258].  Continued use of
   this design pattern lowers the impact of mitigations to that attack.

8.  Contributors

   This document is derived in part from the work initially done on the
   Perpass mailing list and at the [STRINT] workshop.  It has been
   discussed with the IAB's Privacy and Security program, whose review
   and input is gratefully acknowledged.

9.  References

9.1.  Normative References

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <>.

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   [RFC7624]  Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
              Trammell, B., Huitema, C., and D. Borkmann,
              "Confidentiality in the Face of Pervasive Surveillance: A
              Threat Model and Problem Statement", RFC 7624,
              DOI 10.17487/RFC7624, August 2015,

9.2.  Informative References

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <>.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,

   [RFC7239]  Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
              RFC 7239, DOI 10.17487/RFC7239, June 2014,

   [RFC7871]  Contavalli, C., van der Gaast, W., Lawrence, D., and W.
              Kumari, "Client Subnet in DNS Queries", RFC 7871,
              DOI 10.17487/RFC7871, May 2016,

   [STRINT]   S Farrell, ., "Strint Workshop Report", April 2014,

Author's Address

   Ted Hardie


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